Any references in the following discussion to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
Millimeter-wave radio communications apparatus enables the wireless transmission of high speed data between two points. Such apparatus has become an increasingly common method for backhaul of data from cellular networks to a fiber backbone.
U.S. Pat. No. 6,016,313 describes multiplexing a plurality of mm-wave antenna arrays to support transport in such a cellular network. In U.S. Pat. Nos. 6,714,800, 7,062,293, 7,769,347, and 7,912,506 the use of multiple mm-wave radios to assemble networks for cellular backhaul are disclosed. The systems described in these patents rely on the key advantages of mm-wave propagation: large bandwidth, which supports high data rates; and narrow beamwidth, which supports multiple nodes operating from the one site since mutual interference of the radio waves is low. U.S. Pat. No. 7,065,326 describes a mm-wave system with a particular modulation circuit whose half power beam width is about 0.2 degrees or less.
Although mm-wave communications systems can support gigabit-per-second data throughput, they have a susceptibility to atmospheric attenuation which limits communications distances to lengths of no more than several miles, and only in good weather. In U.S. Pat. Nos. 6,556,836 and 6,665,546 there are described gigabit-per-second communications at 95 GHz. However these systems require use of a lower frequency backup transceiver in the event of adverse weather. U.S. Pat. Nos. 6,169,910 and 8,090,411 and international patent publications WO 2013058673 and WO 2014011087 describe mm-wave systems using special dielectric lens antennas, or multiple feed arrays or switchable focal plane arrays used for electronic beam steering to improve communications and achieve alignment under such conditions.
However, as some of these patents describe, the narrow beamwidth at mm-wave frequencies also creates difficulty in correctly aligning two ends of a terrestrial link that are separated by large distances, since the antennas need to point directly at each other to avoid missing each other's narrowly focused beams. U.S. Pat. No. 6,587,699 describes using an optical alignment method to align antennas at either end of a terrestrial link. U.S. Pat. No. 6,611,696 claims an alignment method requiring two installers at each end who initially align the antennas visually, and then use the strength of a transmitted tone to manually fine-tune the alignment.
U.S. Pat. No. 7,680,516 claims an automatic alignment technique in which the antenna is mounted on gimbals, but whose positioning data is obtained from GPS signals and must be shared between the two ends. U.S. Pat. No. 6,307,523 B1 describes an automatic tracking technique specifically for two-way communications with a skyborne target in which a sub-reflector is required in the main path to modulate the main received signal to generate a tracking signal through which the pointing direction may be controlled.
Digital beam forming techniques may also be adapted to indicate the direction of the incoming mm-wave signal and to point the principal axis of the receiver antenna in the correct direction. US patent publication number 20060246863 and international patent publication WO 2011056256 describe such digital beam forming or beam peaking techniques for communications systems, while U.S. Pat. No. 8,558,746 describes the construction of a flat panel array antenna for frequencies below 26 GHz.
Referring now to FIGS. 1 and 2, there are depicted top plan and side views of a terrestrial communications link established by opposed link ends in the form of microwave frequency transceivers and directional parabolic reflector antennas mounted on respective towers.
As previously alluded to, a problem that arises with a system such as that of FIGS. 1 and 2, which is particularly pronounced where the link operates at millimeter wavelength frequencies, is that it is difficult to maintain mutual alignment of the ends of the link. This problem arises because the operating wavelength is so small that even moderate size antennas (of say 1.2 m diameter) have a very narrow beamwidth, which is typically around 0.25 degree in the E-band (75-85 GHz).
Even if the link is set up correctly initially, with both ends in mutual alignment, a variety of factors may cause the ends to misalign. One reason for misalignment occurring is that the towers at each end may tilt or twist due to wind or other forces. Wind-induced motion at either end of the communications link will cause the center of the transmitted signal beam to completely “miss” the remote antenna, causing lack of any received signal at the remote end and a link outage. Such motion can occur at either, or both, ends of a link.
It will be realized that a link outage is highly undesirable and depending on the nature of the traffic being carried it may have very serious ramifications.
In addition to the above problem, it is presently difficult to “fine tune” an initial somewhat coarse alignment of an end of a terrestrial communication link. It would be advantageous if it were possible to subsequently improve upon an initial alignment.